This invention relates to cold forming of plastic substrates, and more particularly, to the cold forming of plastic substrates without the aid of a lubricant.
Thermoplastic polymers such as the olefin polymers, alkyl aromatic resins and the like are conventionally formed into desired shaped articles, such as containers and the like, by injection or compression molding. Also sheets, formed by extrusion or molding of such thermoplastic polymers, may be shaped by such techniques as drape molding or vacuum forming and the like into various useful articles such as cups and generally similar configurations. Such methods call for the heat-plastification of granules or sheets of such materials and subsequent forming of the heat-plastified mass under pressure in a mold or over a form capable of restraining the formed or shaped mass in the desired position until the polymer cools. Conventional molding or forming procedures generally impart to the polymeric material an undesirable heat history which usually decreases the resistance to oxidation and the like. A further disadvantage of hot forming techniques, such as molding and vacuum drawing, is the close temperature control required therein. Hot forming invariably is accompanied by shrinkage problems and the need for precise temperature control. Usually in vacuum drawing operations, the resultant product has non-uniform thickness and frequently areas of greatest draw are undesirably thin.
Inherently, processes involving the heat plastification of polymers are slow and are not readily adapted to high production rates such as may be obtained in metal fabrication, wherein sheet metal may be punched, drawn, and otherwise formed without the temperature of the sheet being raised substantially.
In order to overcome many of the problems arising in hot forming plastic materials, it has been the practice to manufacture hollow articles and other articles of thermoplastic polymers by cold forming processes, such as cold drawing, cold stamping and the like. In accordance with cold drawing techniques, plastic sheets are cold drawn (often by so-called "deep-drawing") into the form of cups, bowls, tubs, tubular articles, and other similar such desired configurations. In accordance with another cold forming process, a resin blank (usually in disk form) is cold extruded by placing the blank between two coaxially opposite mold pieces of mating configuration and then squeezing the resin blank therebetween such that at least part of the resin is extruded and flows between the mold pieces to form the desired shape. Pressure between the mold pieces is subsequently relaxed and the molded article is withdrawn therefrom. Other methods for cold forming thermoplastic polymers at temperatures below the heat plastification point of the polymer are also well known. In order to avoid rupture of the thermoplastic polymer substrate during the cold forming operation, it is a practice in the art to coat the substrate and/or the surfaces of the shaping or drawing means with a lubricant such as an oil, an aqueous oil emulsion, soap water, or even for some resins having marked water repellant character, simply water. Unfortunately, however, there is the accompanying problem of lubricant build-up on the die surfaces of the molding device and/or the formed substrate, and it is usually necessary to remove lubricant several times during the procedure from the article and the dies.
In view of the foregoing problems of the prior art, it would be highly desirable to provide a cold forming technique which would enable the practice of a high speed cold forming method without the requirement of lubricant.